1. Field of the Invention
The present invention relates generally to antennas and, more particularly, to antennas which have a low profile.
2. Related Art
Low profile antennas are known. For example, U.S. Pat. No. 5,327,148 to How et al describes a microstrip antenna that has a substrate that includes a ground plane conductor disposed over a first surface and a strip conductor disposed over a second surface. A DC magnetic field biasing circuit provides a directed DC magnetic field to the substrate such that the strip conductor radiates electromagnetic energy that has a circular polarization. In one particular embodiment, the substrate is composed of magnesium ferrite and in another, a second substrate of ferrite material is disposed over the strip conductor to reduce the radar cross section of the antenna.
The antenna, described by How et al, suffers from the drawback that a significant fraction of energy is dissipated in surface waves because of the limited size of the ground plane.
An effort was made to overcome the foregoing drawback by Daniel Frederic Sievenpiper in his Ph.D. thesis entitled “High-Impedance Electromagnetic Surfaces”, University of California, Los Angeles, 1999 (below referred to as “Sievenpiper”). Sievenpiper describes providing a high impedance surface which reduces surface waves and which consists of a plurality of metal protrusions on a flat metal sheet. The metal protrusions include flat metal plates disposed on vertical posts. Each of the metal plates and posts function to provide a capacitance and an inductance and as such function as electric filters to block the flow of surface waves.
One disadvantage that arises in connection with an antenna employing a relatively high impedance electromagnetic ground plane surface, such as that described by Sievenpiper, is that an associated narrow bandwidth of approximately 8% occurs when transmitting at microwave frequencies. Accordingly, it is desired to provide a low profile antenna that is both efficient and that does not compromise bandwidth.